In explosion at a religious school in the northwestern Pakistani city of Peshawar has killed at least seven people and wounded 109 others, police and health officials say.

The blast occurred at the Speen Jamaat mosque, which also serves as a religious school for the local community in the city’s Dir Colony area, at 8:30 am local time (3:30 GMT) on Tuesday, a police official told Al Jazeera shortly after the blast.

“[Students] were reading the Quran here, that is when the explosion occurred,” Peshawar police chief Muhammad Ali Khan told reporters near the scene.

“The initial investigation shows […] that five to six kilogrammes [11-13 pounds] of explosive material was used [and] that someone came here and left a bag of explosives.”

It was not immediately clear how many children were among those killed or wounded, as the students gathered at the school included many who were adults.

Speaking to local television station Geo News, the provincial police’s bomb disposal unit chief Shafqat Malik said the device used was sophisticated and involved a timed detonation.

“The forensic evidence that we have picked up, shows that it was about 5kg [11 pounds] of explosives and it was a timed device,” said Malik.

“It seems to be a high-quality device, which appears to use. There has been a lot of damage, and this [attack] has been planned with great thought.”

Television footage from the scene of the blast showed significant damage to the interior of the mosque’s main prayer hall, with pockmarks dotting the ceiling and debris strewn across the floor.

At least 83 of those wounded were being treated at the Lady Reading Hospital (LRH), the city’s main government hospital, while 26 others were brought to the Naseerullah Khan Babar hospital.

Tariq Burki, an official at the Lady Reading Hospital, said five of the wounded were in critical condition.

“[We] have referred them to the burns center [for treatment], and two are in the operating room,” he told Al Jazeera by telephone. “Most of the patients have received burn injuries.”

Burki confirmed that there were four children among those wounded and that all of those killed, as well as most of the injured, were aged between 20 and 40.

Nasrullah Khan Babar hospital official Shafiq-ur-Rehman told Al Jazeera by telephone that the wounded brought to them had “almost all been discharged by now”.

“They all had minor injuries, fractures, and the such,” he said.

There was no immediate claim of responsibility following the attack.

Pakistan has battled the Tehreek-e-Taliban Pakistan (TTP), or Pakistan Taliban, since 2007 when the group was formed and held sway over several districts, carrying out frequent attacks against civilian and security targets across the country.

Violence has sharply declined since 2014 when the Pakistani military launched a series of operations to displace the TTP from its erstwhile headquarters in the country’s northwest, forcing many fighters and commanders to allegedly move into neighboring Afghanistan.

Since 2017, the frequency of bombings like Tuesday’s attack has lessened, but sporadic attacks aimed at civilians and security forces continue.

On Sunday, at least three people were killed after explosives planted in a motorcycle in the southwestern city of Quetta exploded in a market.

That attack was claimed by the Baloch Liberation Army (BLA), an ethnic Baloch separatist armed group. It operates in Balochistan province and wants independence for that region.

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